Celestial Sphere - Celestial Coordinate Systems

Celestial Coordinate Systems

These concepts are important for understanding celestial reference systems, the methods in which the positions of objects in the sky are measured. Certain reference lines and planes, as projected onto the celestial sphere, form the basis of the reference systems. These include the Earth's equator and axis, and the Earth's orbit. At their intersections with the celestial sphere, these form the celestial equator, the celestial poles and the ecliptic, respectively. As the celestial sphere is considered infinite in radius, all observers see the celestial equator, celestial poles and ecliptic at the same place against the background stars.

Directions toward objects in the sky can be quantified by constructing, from this basis, celestial coordinate systems. Similar to the terrestrial longitude and latitude, the equatorial system of right ascension and declination specifies positions relative to the celestial equator and celestial poles. The ecliptic system of celestial longitude and celestial latitude specifies positions relative to the ecliptic. This list of coordinate systems is not exhaustive.